I'mot sure what would be involved in rebuilding a motor? I definitely opened them up, cleaned them, checked the brushes, and re-done the wires if needed.
There is a great tutorial here
http://vssmb.blogspot.com/2012/01/co...ted-motor.html
Even though it's for the potted motor on a Singer 15 or 201 there is great information to get you started.
If you search this site you will find information and photos on more of a standard "universal" type motor.